The present invention relates to a method and apparatus for improving the coating of flat or curved planar surfaces of substrates with liquid coatings, and in particular to coating surfaces having a plurality of fragile members adhered thereto and projecting therefrom without damaging these members.
Using flat field emission display devices as an illustrative example, it is important that uniform spacing be maintained between the cathode electron emitting surface (also referred to as the base electrode, baseplate, emitter surface, or cathode surface) and its corresponding anode display face (also referred to as an anode, cathodoluminescent screen, display screen, faceplate, or display electrode) in order to have proper operation and to prevent catastrophic electrical breakdown, there being a relatively high voltage differential (e.g., generally above 300 volts) between the cathode emitting surface and the display screen. At the same time, it is necessary to maintain the narrow spacing between the cathode and anode and to make these from relatively thin materials to achieve the desired structural thinness and high image resolution and brightness, as well as to avoid display distortion, etc. Uneven spacing between the electron emitting surface and anode is much more likely to occur in a field emission cathode, matrix addressed, flat vacuum type display than in other types of displays because of the narrow spacing and the thin materials used to form the baseplate and the faceplate. Spacers play an important role in maintaining the required uniform spacing and structural integrity of the large area, light weight, flat panel, field emission displays.
Spacers are incorporated between the faceplate and the plate upon which the emitter tips are fabricated. The spacers can be formed on a substrate by any of a number of methods, such as screen printing and stencil printing; by reactive ion etching and plasma etching of deposited materials; and by adhering a plurality of preformed spacers at defined sites on the substrate. The spacers formed by these methods must not be either too short for the high voltages encountered, or too wide as to interfere with the display image.
The application of thin coatings of coating materials, i.e., less than about 10 microns thick coatings, has become an increasingly important step in the manufacture of various products including, but not limited to: the above-mentioned flat panel displays. This is particularly true when the substrate includes a surface to be coated which has an array of fragile members adhered to and extending therefrom. These fragile members could include such things as the above discussed spacers, which might be damaged, destroyed, or removed by a coating process, to the ultimate degradation of the final product.
In a pre-metered coating process the process liquid, such as photoresist, developer, etchant, chemical stripper, solder mask, or any other liquid chemical can be coated onto the surface of a substrate. The term pre-metered coating refers to a process in which a controlled volumetric flow rate of coating liquid is fed into a coating applicator that is spaced a distance from the surface of a substrate to be coated. As the substrate passes by the applicator, the coating liquid issuing from the applicator is deposited onto the surface as a thin uniform layer.
Retained coating processes, such as dip coating, spin coating, screen coating or roll coating, expose the substrate's surface to an excess supply of coating liquid. When that excess coating liquid is removed, the amount retained on the substrate surface is a function of the coating fluid's rheological properties, (viscosity, etc.) and the coating process parameters. Generally, a liquid's viscosity will change if its percentage of solids changes either by loss of the coating fluid's solvent through evaporation or by over dilution of the coating liquid when solvent is back added to compensate for evaporation losses.
Spin coating is often used for applying resist coatings, but would be highly unsuitable in the present case as the centrifugal forces developed would damage the fragile members. There also is the possibility of streaking of the coated film due to interruption of the liquid path. Roll coating would also be unacceptable as it would clearly crush the fragile members. Dip coating and simpler coating methods provide less destructive methods for applying the coating material. However, the coating thickness and reproduciblity of the coating can be difficult to control, particularly when the surface to be coated has thereon members which will affect both the retention of and the runoff of the coating liquid.
Meniscus coating of an object, such as the above-mentioned substrate, is a known coating technique. In this process, a coating material is passed through a permeable surface of an applicator so as to develop a downward laminar flow of coating material on the outer surface of the applicator. An object is advanced so that the surface to be coated tangentially intersects the laminar flow of coating material near the apex of the applicator. Menisci of flowing coating material are formed both at the leading and the trailing edges of the coating material in contact with the surface to be coated and assure the uniform disengagement and drainage of excess coating material from the coated surface to the downward laminar flow of coating material on the outside surface of the applicator.
Capillary coating comprises placing the lower ends of two parallel, closely spaced plates into a reservoir of coating fluid so that, by capillary action, the coating fluid rises between the plates and forms a meniscus at the top opening between the plates. The substrate is then brought into contact with and moved relative to the meniscus so that a layer of coating material forms on the substrate. This method provides a fairly uniformly thick layer because the flow of coating material onto the substrate is adequately controlled by the capillary action. However, this method is usually limited to coating fluids having a viscosity of less than thirty centipoises.
Slot coating comprises an extrusion method in which a viscous coating material, under positive pressure, is passed through a thin elongated orifice that extends across and is in close proximity to, but spaced apart from, the surface of the substrate to be coated. A thin flexible continuous film is extruded from the orifice and deposited on the surface to be coated as it is moved linearly past the orifice so as to uniformly deposit the thin film thereon. Coatings of less than 100 microns can be deposited by this process.
Patch coating comprises dispensing a pre-metered amount of coating material onto a substrate with a pre-configured layer of coating liquid in controlled volume of liquid per unit area of substrate. The liquid is dispensed from an applicator slot coupled to a containment chamber in which the volume of liquid is varied in order to sharply start and stop the patch by producing a pulse of liquid. The substrate is moved relative to the applicator slot, at least during the period between formation and termination of the bead of liquid in order to produce a patch of the desired dimensions and uniform thickness.
Patents relating to meniscus coating include U.S. Pat. Nos. 4,370,356; 5,270,079; and 5,368,645. U.S. Pat. No. 4,938,994 relates to patch coating while U.S. Pat. No. 4,696,885 relates to capillary or slot coating. The disclosures of all of these patents are incorporated herein by reference.